1. Field of the Invention
The present invention is directed to a magnetic resonance signal evaluation method of the type wherein reception signals are simultaneously supplied from a reception volume by a number of antenna elements of an array antenna for a magnetic resonance apparatus, wherein the reception signals are combined with one another in a basic combination, so that the basic combination reproduces a circularly polarized magnetic resonance signal, and wherein the basic combination is utilized for image reconstruction.
The present invention also is directed to a reception arrangement for a magnetic resonance apparatus of the type having an array antenna with a number of antenna elements with which reception signals are simultaneously received from a reception volume, wherein the reception signals are supplied to a combination element with which the reception signals are combined with one another in a basic combination that reproduces a circularly polarized magnetic resonance signal, and wherein the basic combination is supplied to an image reconstruction element that utilizes the basic combination for image reconstruction.
2. Description of the Prior Art
Signal evaluation methods and corresponding reception arrangements of the type are well known. In particular, antennas referred to as birdcage resonators and loop-butterfly elements operate according to this principle.
A very good signal-to-noise ratio is generally required in the evaluation of magnetic resonance signals. In order to achieve this good signal-to-noise ratio, it is known to evaluate a number of reception signals.
An object of the present invention is to provide a signal evaluation method and a reception arrangement corresponding therewith with which an optimum signal-to-noise ratio can be achieved with as little outlay as possible.
This object is achieved in accordance with the invention in an evaluation method of the above type wherein the reception signals are also combined with one another in a number of auxiliary combinations, with all combinations being orthogonal to one another and wherein the auxiliary combinations contain components derived from circularly polarized magnetic resonance signal components of the magnetic resonance reception signals that are oriented isodirectionally to the basic combination, and wherein at least two combinations are utilized for the image reconstruction.
This object is achieved in accordance with the invention in a reception arrangement of the above type wherein the combination element also combines the reception signals with one another in a number of auxiliary combinations, the combinations being orthogonal to one another, with the auxiliary combinations containing circularly polarized magnetic resonance signal components that are oriented isodirectionally to the basic combination, the combinations being supplied to an image reconstruction unit, and wherein at least one of the auxiliary combination is utilized by the image reconstruction unit for image reconstruction in addition to the basic combination.
The noise in the individual combinations is uncorrelated due to the orthogonally of the combinations. The individual combinations thus can be quadratically added without difficulty. The orthogonality of two combinations is defined in that they satisfy the equation
∫"sgr"E1E2dV=0
within the reception volume, wherein "sgr" is the conductivity of human tissue and E1 and E2 are the electrical fields induced by the respective antenna elements of one of the combinations if the antenna elements were transmitting.
The combinations usually exhibit reception sensitivities that differ from one another. The signal-to-noise ratio therefore can be optimized when the combinations are utilized for the image reconstruction weighted with their respective reception sensitivities.
When the basic combination exhibits a reception sensitivity differing from zero in the center of the reception volume, a good image reconstruction results, particularly in the center of the reception volume. When the basic combination exhibits a substantially spatially independent reception sensitivity in the reception volume, then an image reconstruction over the entire reception volume is always possible, even independently of the number of auxiliary combinations utilized for the image reconstruction.
The auxiliary combinations usually exhibit a reception sensitivity of zero in the center of the reception volume. This means the edge regions of the reception volume can be reconstructed well with the respective combinations. This is especially true when the auxiliary combinations exhibit a radially increasing reception sensitivity in the reception volume with reference to a symmetry axis. The reception sensitivity of one of the auxiliary combinations should increase essentially linearly with the distance of the observed location from the symmetry axis.
For example, the reception signals can be azimuthally acquired with reference to a basic magnetic field direction. In this case, at least four reception signals must be acquired.
It is also possible to longitudinally acquire the reception signals with reference to a basic magnetic field direction or a radiofrequency field direction. In these cases, it suffices when at least two reception signals are acquired.
The antenna elements should be resonant at the Larmor frequency for all combinations utilized for the image reconstruction. It is therefore required that the antenna elements be inductively-capacitatively decoupled from one another in pairs. The inductive-capacitative decoupling can be achieved, for example, by providing the antenna array with a capacitor network for this purpose.
The combination element can be fashioned as a Butler matrix, allowing it to be realized in an especially simple way.
The reception arrangement can have a transmitter allocated to it that is connected to the antenna elements via a distributor element, and a transmission signal emitted by the transmitter can be divided among the antenna elements by the distributor element so that a magnetic resonance excitation signal is fed into the reception volume, so a subject arranged in the reception volume can be excited to magnetic resonance with the array antenna.
The distributor element can be a component of the Butler matrix, making the structure of the distributor element especially simple.
When the magnetic resonance excitation signal has an essentially location-independent excitation intensity in the reception volume, essentially the entire reception volume is exposed to the magnetic resonance excitation signal.